Light emitting diode technologies have been significantly changed by development of GaN-based light emitting diodes, which are currently applied to various fields, such as full color LED displays, LED traffic signal systems, white LEDs, etc.
Recently, it is expected that highly efficient white LEDs will replace fluorescent lamps, and efficiency of white LEDs is approaching the level of typical fluorescent lamps. However, there is still a need for further improvement in the efficiency of LEDs.
In order to improve the efficiency of LEDs, two main approaches are proposed in the art: one is to increase internal quantum efficiency that is determined by crystal quality and epitaxial layer structure, and the other is to increase light extraction efficiency.
As a technique for increasing the light extraction efficiency, there is a patterned sapphire substrate-based technique, as described in “InGaN-based Near-Ultraviolet and Blue Light Emitting Diodes with High External Quantum Efficiency Using a Patterned Sapphire Substrate and a Mesh Electrode” (Japanese Journal of Applied Physics, Vol. 41, pp. L1431-L1433), published on Dec. 15, 2002).
According to this technique, a sapphire substrate is etched to have a pattern of convex hexagons and is then formed with nitride semiconductor layers to manufacture a light emitting diode, which may have improved light extraction efficiency through reduction of light loss caused by total internal reflection between the nitride semiconductor layers and the substrate.
However, the patterned sapphire substrate has many growth planes, and, when nitride semiconductors grown on the respective growth planes meet together to form the semiconductor layer, a relatively large amount of defects such as pin holes can be formed in the semiconductor layer. The pin holes disadvantageously affect growth of nitride semiconductor layers, for example, active layers, to be formed on the semiconductor layer, and reduce internal quantum efficiency of LED while causing current leakage during operation of the LED, thereby deteriorating brightness and reliability of the LED.
To solve the problems as described above, a method of continuously growing a nitride semiconductor layer on a patterned sapphire substrate in 3D and 2D growth conditions is generally used.
FIG. 1 is a cross-sectional view illustrating a conventional method of forming a nitride semiconductor layer on a patterned sapphire substrate.
Referring to FIG. 1, a nucleation layer 13 is formed on a patterned sapphire substrate 11. The substrate 11 is processed to have a pattern of islands 11a and a substantially flat base surface on a recess region between the islands 11a. The nucleation layer 13 is preferentially grown on the flat base surface and covers the base surface.
Then, a 3D growth layer 15 is formed on the nucleation layer 13 of the substrate 11 by growing a nitride semiconductor layer thereon in a 3D growth condition. Here, the term “3D growth condition” means a condition wherein vertical growth is faster than lateral growth due to a lower mobility of atoms during growth of the nitride semiconductor layer. The 3D growth layer 15 fills the recess regions on the substrate 11 and covers the islands 11a. 
Then, a 2D growth layer 17 is formed on the 3D growth layer 15 by growing a nitride semiconductor layer thereon in a 2D growth condition. Here, the term “2D growth condition” means a condition wherein lateral growth is faster than vertical growth due to a higher mobility of atoms during growth of the nitride semiconductor layer. On the 2D growth layer 17, a first conductive semiconductor layer, an active layer, and a second conductive semiconductor layer (not shown) are grown, thereby forming a light emitting diode. Here, the nucleation layer 13, 3D growth layer 15, and 2D growth layer 17 can be formed by MOCVD.
According to the conventional technique, since the 2D growth layer 17 is formed on the 3D growth layer 15 filling the recess regions on the substrate surface, the 2D growth layer 17 can be grown without any influence of the growth planes on the substrate. Accordingly, it is possible to suppress generation of the pin holes, which could be formed when the 2D growth layers continue to grow and meet one another during growth of the 2D growth layer 17 directly on the nucleation layer 14. However, since the 3D growth layer 15 must be formed relatively thick to fill the recess regions of the substrate 11, the 3D growth layer 15 has a significantly rough surface. When the 2D growth layer 17 is formed on the 3D growth layer 15 having the rough surface, lots of pin holes are formed not only on an interface between the 3D growth layer 15 and the 2D growth layer 17, but also inside the 2D growth layer 17.
The present invention is conceived to solve the problems of the conventional techniques as described above, and an aspect of the present invention is to provide a method of forming a high quality nitride semiconductor layer on a patterned substrate.
Another aspect of the present invention is to provide a method of forming a nitride semiconductor layer on a patterned substrate, in which the nitride semiconductor layer can suppress generation of pin holes.
A further aspect of the present invention is to provide a light emitting diode, which has nitride semiconductor layers formed on a patterned substrate to ensure high brightness and reliability of the light emitting diode, and a method of manufacturing the same.
In accordance with one aspect of the present invention, a method of forming a nitride semiconductor layer on a patterned substrate is provided. The method comprises preparing a patterned substrate having a pattern of protrusions and recess regions. A nucleation layer is formed on the patterned substrate and a first 3D growth layer is formed on the substrate having the nucleation layer by growing a nitride semiconductor layer in a 3D growth condition. Then, a first 2D growth layer is formed on the first 3D growth layer by growing a nitride semiconductor layer in a 2D growth condition. Next, a second 3D growth layer is formed on the first 2D growth layer by growing a nitride semiconductor layer in a 3D growth condition, and a second 2D growth layer is formed on the second 3D growth layer by growing a nitride semiconductor layer in a 2D growth condition.
Here, the term “3D growth condition” means a condition wherein vertical growth is faster than lateral growth due to a lower mobility of atoms during growth of the nitride semiconductor layer. In the 3D growth condition, the nitride semiconductor layer grows fast in the vertical direction and has a rough surface. On the other hand, the term “2D growth condition” means a condition wherein lateral growth is faster than vertical growth due to a higher mobility of atoms during growth of the nitride semiconductor layer. In the 2D growth condition, the nitride semiconductor layer grows so as to have a flat surface.
According to embodiments of the present invention, the 3D growth condition for growing the first 3D growth layer is set to have a substrate temperature (T1) in the range of 600˜1,200° C. and a chamber pressure (P1) in the range of 10˜760 mbar. Further, the 2D growth condition for growing the first 2D growth layer is set to have a substrate temperature (T2) higher than T1 and a chamber pressure (P2) lower than P1. Further, the 3D growth condition for growing the second 3D growth layer is set to have a substrate temperature (T3) lower than T2 and a chamber pressure (P3) higher than P2. Additionally, the 2D growth condition for growing the second 2D growth layer is set to have a substrate temperature (T4) higher than T3 and a chamber pressure (P4) lower than P3.
According to the embodiments of the present invention, since the thickness of the 3D growth layer can be reduced by alternately forming the 3D growth layers and the 2D growth layers, it is possible to prevent the 3D growth layers from having rough surfaces and to thereby improve crystal quality of a final 2D growth layer.
Further, even when a 3D growth layer is formed thick and has a rough surface, a 2D growth layer and another 3D layer are previously formed before the final 2D growth layer, thereby preventing pin holes from being formed in the final 2D growth layer due to the rough surface thereof.
In the meantime, the first 3D and 2D growth layers may be formed in the recess regions of the substrate to have upper surfaces located below the pattern of protrusions. In addition, the second 3D growth layer may be formed to have an upper surface located at least above the pattern of protrusions. Unlike the conventional technique where the recess regions of the patterned substrate are filled with a single 3D growth layer, the present method fills the recess regions of the patterned substrate with the 3D growth layer and 2D growth layer, thereby preventing the 3D growth layer from being formed to have a rough surface.
The present method may further comprise alternately forming an additional 3D growth layer and an additional 2D growth layer at least once on the substrate, on which the first 2D growth layer is formed, before forming the second 3D growth layer. In other words, the 3D growth layers and 2D growth layers may be repetitively grown, thereby preventing the 3D growth layers from having the rough surface even when the depths of the recess regions of the patterned substrate increase.
A growth condition for the additional 3D growth layer is set to have a lower substrate temperature and a higher chamber pressure than those for the 2D growth layer formed under the additional 3D growth layer, and, a growth condition for the additional 2D growth layer is set to have a higher substrate temperature and a lower chamber pressure than those for the additional 3D growth layer formed under the additional 2D growth layer.
A first conductive semiconductor layer, an active layer, and a second conductive semiconductor layer may be formed on the second 2D growth layer. Then, a light emitting diode is manufactured by patterning the first conductive semiconductor layer, active layer, and second conductive semiconductor layer.
In accordance with another aspect of the present invention, a light emitting diode comprises a substrate that is patterned to have a pattern of protrusions and recess regions. The light emitting diode comprises a nucleation layer covering base surfaces of the recess regions of the substrate. A first 3D growth layer of a nitride semiconductor layer grown in a 3D growth condition is positioned on the nucleation layer, and a first 2D growth layer of a nitride semiconductor layer grown in a 2D growth condition is positioned on the first 3D growth layer. Further, a second 3D growth layer of a nitride semiconductor layer grown in a 3D growth condition is positioned on the first 2D growth layer, and a second 2D growth layer of a nitride semiconductor layer grown in a 2D growth condition is positioned on the second 3D growth layer.
The first 3D growth layer is the nitride semiconductor layer grown on the nucleation layer in a 3D growth condition set to have a substrate temperature (T1) in the range of 600˜1,200° C. and a chamber pressure (P1) in the range of 10˜760 mbar. Further, the first 2D growth layer is the nitride semiconductor layer grown on the first 3D growth layer in a 2D growth condition set to have a substrate temperature (T2) higher than T1 and a chamber pressure (P2) lower than P1. Further, the second 3D growth layer is the nitride semiconductor layer grown on the first 2D growth layer in a 3D growth condition set to have a substrate temperature (T3) lower than T2 and a chamber pressure (P3) higher than P2. Additionally, the second 2D growth layer is the nitride semiconductor layer grown on the second 3D growth layer in a 2D growth condition set to have a substrate temperature (T4) higher than T3 and a chamber pressure (P4) lower than P3.
Additionally, a first conductive semiconductor layer is positioned on the second 2D growth layer and a second conductive semiconductor layer is positioned on the first conductive layer. Further, an active layer is interposed between the first and second conductive semiconductor layers.
According to this aspect of the present invention, since the second 2D growth layer having excellent crystal quality is provided to the light emitting diode, the semiconductor layers formed on the second 2D growth layer also have improved crystal quality, thereby ensuring the light emitting diode has excellent internal quantum efficiency.
In the meantime, the first 3D growth layer and the first 2D growth layer may be positioned in the recess regions of the substrate, with their upper surfaces located below the pattern of protrusions. In addition, an upper surface of the second 3D growth layer may be located at least above the pattern of protrusions. Accordingly, the thickness of the 3D growth layer can be reduced by filling the recess regions of the patterned substrate with the 3D growth layer and the 2D growth layer, thereby enabling reduction in surface roughness of the 3D growth layer.
Moreover, at least one pair of additional 3D and 2D growth layers may be interposed between the first 2D growth layer and the second 3D growth layer. The additional 3D growth layer is formed at a lower substrate temperature and a higher chamber pressure than those for the 2D growth layer formed under the additional 3D growth layer, and the additional 2D growth layer is formed at a higher substrate temperature and a lower chamber pressure than those for the additional 3D growth layer formed under the additional 2D growth layer. Therefore, it is possible to further reduce the thickness of the 3D growth layer.